The invention relates to a novel device and a novel method for packaging in block form a sheathed hot-melt adhesive product.
Various ultrasonic welding devices are known.
Ultrasonic waves are waves with frequencies typically ranging between 15 kHz and a few hundred megahertz. High-intensity ultrasonic waves are able to alter the medium in which they propagate, with repercussions on the physicochemical properties of the medium.
An ultrasonic weld normally requires a transducer, a sonotrode and an anvil. The transducer produces ultrasound from which ultrasonic vibrations are generated. If applicable, a speed transformer amplifies these vibrations and transmits them to a sonotrode, which generally takes the form of a metal finger that can be vibrated at ultrasonic frequency. Layers of elements to be welded are generally placed between the anvil and the sonotrode. The hammering experienced locally transforms the elements and permits local interpenetration of the surfaces of these elements. The temperature rises, the deformation zone extends, and the weld is achieved. The weld is typically obtained in a few tenths of a second.
Routine applications using ultrasound include the cutting and slicing of soft products, stripping, the perforation of thin products, and the welding of hot-melt materials.
Various devices for packaging hot-melt adhesives are also known.
For example, in WO-A-94/13451, a mass of hot-melt adhesive is packaged directly by pouring or pumping the molten adhesive into a cylindrical thermoplastic tube, the cylindrical tube being in contact with a heat sink.
The packaged article resulting from this operation is supplied in the form of a handable bag or block which can be manufactured in an in-line operation.
In particular, the adhesive-filled tube is passed through rollers that press or pinch the filled cylinder in order to form bags of appropriate length. A liquid refrigerant is sprayed until the packaging is sufficiently cooled, so that the packaged adhesive forms a seal at the pressed or pinched portions. The sets of bags can then be cut at the pressed portions using conventional means, for example mechanical shears, laser, water jet, knife or hot wire, and then cooled to ambient temperature.
Document EP-A-0 957 029 discloses a method and an installation for packaging an adhesive product and similar products, the liquid adhesive product being sheathed in a protective sheath of a nonadhesive thermoplastic material. The adhesive product is sheathed by a coextrusion method. The coextrusion product thus formed, composed of the adhesive product and the protective sheath, is, at least as regards the protective sheath, solidified.
The advantage of sheathing the adhesive product in a nonadhesive sheath resides in the possibility of handling (particularly for transport, storage or packaging) or using the product after its manufacture.
It is also known to use a sheath that is miscible with the hot-melt adhesive at a given temperature. In this way, the product and its sheath can be melted together before application, without the need to remove the sheath.
However, the methods and devices described above are not compatible with the packaging of bags or blocks weighing more than 0.5 kg, and for certain compositions and sheath thicknesses. In fact, the heat and pressure exerted by the hot material in the sheath of the block formed are detrimental to the quality of the seal of the sheath. Typically, when the mass of sheathed adhesive exceeds the above values, the seal of the sheath yields (gaping occurs) at certain places. The adhesive product then comprises sticky zones that are detrimental to the subsequent handling or use of the product.